We report on the detection of a fast X-ray transient EP240315a at 2024-03-15T20:10:44 (UTC) by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission during a calibration observation. The position of the source is R.A. = 141.644 deg, DEC = -9.547 deg (J2000) with an uncertainty of 3 arcmin (radius, 90% C.L. statistical and systematic). The light curve of the source shows a multi-peak profile. The transient event lasts for ~1600 seconds and has a peak flux of ~3e-9 erg/s/cm^2 in the 0.5-4 keV band. The averaged spectrum can be fitted by an absorbed power-law with NH = 1.5(-0.9/+1.0) x 10^21 cm^-2 and a photon index of 1.7(-0.4/+0.4). The derived average unabsorbed 0.5 - 4.0 keV flux is 5.3(-0.7/+1.0) x 10^-10 erg/s/cm^2. However, we note that the derived source parameters may be subject to larger uncertainties than those quoted here since in-orbit calibration of the instrument is still in progress.

No previously known bright X-ray sources have been found within the 3 arcmin region around the source position, with only one optical counterpart of white dwarf candidate located at the distance of ~380 pc. Based on the shape and timescale of the observed flare light curve, we tend to consider that the source is not a stellar flare, although this cannot be ruled out. A Swift target of opportunity observation has been proposed, and further follow-up observations are strongly encouraged to identify the nature of this X-ray flare.

The above observation was made with the partially activated WXT instrument during its commissioning phase.

The preliminary results have been posted at Astronomer's Telegram as GCN 35931.  Click here for more EP/LEIA Astronomer's Telegrams.

Related GCN/ATel reports: 

X-ray transient EP240315a: ATLAS detection of a possible optical counterpart AT2024eju

X-ray transient EP240315a: VLT/X-shooter spectroscopic redshift of z = 4.859